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Re: [Discuss-gnuradio] Re: Mode S and ADS-B

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] Re: Mode S and ADS-B
Date: Mon, 16 Apr 2007 20:24:46 +0200
User-agent: Debian Thunderbird 1.0.2 (X11/20070113)

Hew How Chee wrote:
> Hi Martin,
>>To have the most fun, you also really want a tuner
>>with a 8 Mhz wide IF at the right IF-freq.
>>For now I get the best results when using an
>>external microtune tuner with a 5.75 Mhz IF.
> Is the Microtune tuner still available in the market ?
> Or are there any other alternatives (tuner with low IF
> output)
They are very obsolete. I know Matt has been able to get a few, but he is also 
running out of them.

You don't need a low-IF output as long as the frequency of the IF is not near a 
multiple of the niquist freq (half the sampling freq)

What you DO need is a large signal of the IF output. 1 Volt pp would be ideal.

I have been looking at some Philips alternatives (I don't know the types/names 
out of my head.) which look promising.

>>I have been using the code with an external tuner
>>frontend and did some experiments with fm-receiving,
>>which just works fine.
> Interested to know what is the speed of the CPU that
> you are using and how many percent of the CPU power
> has been used up.
I have a dual-core AMD athlon 64 X2 3800+ (2x2 Ghz)
I think only one CPU is used by gnuradio.
I haven't looked at the processor percentages  in detail.

I did have to relax the channelfilter-parameters to get it to run in realtime.
(Ideally the filter should be a 100 kHz low-pass which results in a 200 kHz 
wide channel)

When using 27 MSPS sampling rate and you want a 100 kHz lowpass you get an 
enourmous amount of filtertaps.
By widening the filter and relaxing the max-ripple you get away with less 
filtertaps in exchange for a too wide filter.
This results in less channel selectivity.

This is what you want:
          chan_filt_coeffs = optfir.low_pass (1,           # gain
                                              cxadc_rate,   # sampling rate
                                              80e3,        # passband cutoff
                                              115e3,       # stopband cutoff
                                              0.1,         # passband ripple
                                              60)          # stopband 
This is the best my processor can do:
          chan_filt_coeffs = optfir.low_pass (1,           # gain
                                              cxadc_rate,   # sampling rate
                                              80e3,        # passband cutoff
                                              1.5*115e3,       # stopband cutoff
                                              10*0.1,         # passband ripple
                                              60)          # stopband 

What also works for me is:
          chan_filt_coeffs = gr.firdes.low_pass         (       1,
                125.0e3, #cutoff (you would want 100.0e3)
                50.0e3   #transition wide, you would want 20.0e3 to 40.0e3 here

I know that a few years back when I was experimenting with a bt878 as capture 
source (also 27 MSPS) I also ran into this.
Then I had a AMD duron 800 Mhz
For that processor I had to widen the filter to 1.6 Mhz to get it to run in 

This is when I started my attemt to build a GPGPU decimating FIR-filter to run 
the filter on my videocard.
I never submitted the code because the results were a bit disappointing.
This is mainly because FIR-filters on a GPU are very much bandwidth limited and 
my videocard only has a memory bandwidth of 6 Mbyte/sec.
You also have the AGP  bottleneck of getting data onto the videocard.

With current videocards this could be a viable option again because current 
videocards have much higher memory bandwith and PCI-express is much
better at transporting data to and from the videocard (readback is much better).

So I think a TV-capture card with a GPGPU decimating freq-translating 
fir-filter and for the rest the normal gnuradio distribution would run in
realtime on a lot of machines if they have a fast videocard.

> Trying to gauge whether with direct
> ADC sampling, are there still enough CPU power to run
> FM-stereo and RDS decoder in real time.
> Regards,
> How Chee
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